Processing stabilizer formulations

ABSTRACT

Melt extrusion processes involving phosphite stabilized polyolefin compositions subject to solid by-product formation during melt extrusion are improved by the addition of a metal salt of a lactic acid to the composition. The processes involving the improved compositions exhibit reduced levels of screen pack plugging during fiber and film extrusion processes than that achieved with fatty acid salts, such as calcium stearate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a melt extrusion processes, and moreparticularly relates to melt extrusion processes for makingpolypropylene fibers or films.

2. Background of the Invention

Processes for making polymeric fibers and films are known, see U.S.Patent Nos. Knox, U.S. Pat. No. 4,156,071, issued May 22, 1979,Frankfort et al, U.S. Pat. No. 4,134,882, issued Jan. 16, 1979, Piazzaet al., U.S. Pat. No. 3,772,872, issued Nov. 20, 1973, Petrille, et al.,U.S. Pat. No. 3,771,307, issued Nov. 13, 1973, Kilian, U.S. Pat. No.3,002,804, issued Oct. 3, 1961, Coates et al., U.S. Pat. No. 2,957,747,issued Oct. 25, 1960, Hebeler, U.S. Pat. No. 2,604,667, issued Jul. 29,1952, all of which are incorporated herein by reference, and GreatBritain Patent Nos. 903427, published Aug. 15, 1962, 1487843, publishedOct. 5, 1977 and 1574305, published Sep. 3, 1986, all of which areincorporated herein by reference. Phosphites are known stabilizationadditives for polyolefins, see York, U.S. Pat. No. 4,305,866, Lewis,U.S. Pat. No. 4,403,053, issued Sep. 6, 1983 and Valdiserri et al, U.S.Pat. No. 4,302,383, issued Nov. 24, 1981, all of which are incorporatedherein by reference.

Polyolefin processors are attempting to process polymer at increasingtemperatures and with increased shear and work on the polymer. They arealso processing polymer which may contain polymerization catalystresidues. The total residual metal content has been decreasing in recentyears but the catalyst residue may still be active.

This combination of more abusive processing conditions and thepossibility of catalyst residue still being active may lead todifficulties when trying to process the polymers.

Catalyst "neutralizers" are well known in the art and are generally usedin most formulations to inhibit corrosion of processing equipmentresulting from catalyst residues. Typical examples would be: Ca, Zn, orMg Stearates, Ca, Zn, or Mg oxides and synthetic hydroltalcitecompositions such as a product manufactured and sold by Kyowa as DHT4A.Additionally products such as the Calcium Stearoyl Lactates and CalciumLactates have been shown to be beneficial in catalyst neutralization asevidenced by reduced corrosion.

In many of the high temperature melt processes such as fiber spinningand film manufacture, screen packs are utilized to remove smallparticles which may be in the polymer prior to the polymer passingthrough the small orifices used in fiber and film processes. With thehigher processing temperature/high shear applications there is atendency for some combinations of polymers and additive formulations tobe prone to screen pack plugging.

Specifically, it has been discovered, however, that stabilizedpolyolefin compositions containing residual catalysts, can generatesolid byproducts during melt extrusion processes. These solids must befiltered out from the melt stream.

For example, melt stream fiber forming processes and film formingprocesses or the fiber and/or film forming dies will become clogged orthe final articles (films/fibers) will exhibit defects and blemishes.Too much solid generation will lead to frequent filter clogging,referred to as screen pack plugging, which leads to increased processingpressures and reduced process throughput.

Consequently, there is a need for improved polyolefin compositions andimproved melt extrusion processes that will exhibit reduced solidbyproduct formation and a resulting reduced filter clogging and areduced increase in processing pressure and improve in throughputs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the process of the present inventionfor making fibers, and

FIG. 2 is a schematic drawing of the process of the present inventionfor making films.

SUMMARY OF THE PRESENT INVENTION

The present invention involves an improved polyolefin melt extrusionprocess that exhibits reduced filter clogging. The process involves (a)forming a polyolefin composition comprising a polyolefin resin, aphosphite stabilizer, a metal salt of a lactic acid and optionally aprimary antioxidant, (b) melt extruding the composition through a filterto produce a filtered melt stream, and (c) passing the melt streamthrough a die to make the plastic article. The utilization of the metalsalt of a lactic acid results in reduced filter clogging.

DETAILED DESCRIPTION OF THE INVENTION

The olefin polymers contemplated herein include homopolymers andcopolymers of monoolefins, preferably those monoolefins containing 1-4carbon atoms. Illustrative examples include polyethylene (including lowdensity, high density, ultra high molecular weight and linear lowdensity polyethylene), polypropylene, EPDM polymers, ethylene-propylenecopolymers and polyisobutylene. The stabilization of mixtures of any ofthese olefin polymers and copolymers likewise is contemplated.

Any polypropylene resin melt extrusion process involving polymerfiltration can be improved by the process of the present invention,including propylene homopolymers and random or block copolymers ofpropylene and an α-olefin which contain ethylene or other α-olefin in anamount from 1 to 30 wt. % as well as blends of polypropylene with otherolefin polymers and copolymers, such as low and high densitypolyethylene, ethylene/vinyl acetate copolymer, ethylene/propylenecopolymer rubbers and styrene/butadiene block-copolymer rubbers.

Phosphites may be replaced in whole or in part with a phosphonite. Thecompositions preferably employ a phosphorous containing componentselected from the group consisting oftetrakis(2,4-di-t-butyl-phenyl)4,-4'-biphenylylene diphosphonite,tris(2,4-di-t-butylphenyl)-phosphite, trisnonylphenyl phosphite,bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite optionally with 1%tri-isopropyl amine, bis(distearyl)pentaerythritol diphosphite, andbis(distearyl)pentaerythritol diphosphite with one percent (1%)triethanolamine.

Phosphites may also be referred to as organic phosphite esters.

The organic phosphite ester is preferably a pentaerythritol diphosphitewhich in most instances is characterized by a spiro structure, i.e.,##STR1## where R is an organic radical. Particularly preferred radicals(for R) are alkyl and alkylphenyl. When R is alkyl it should contain 10to 20 carbon atoms, inclusive, and an especially desirable phosphite isdistearyl pentaerythritol diphosphite, when R is alkylphenyl the alkylsubstituents should contain 3 to 10 carbon atoms and, preferably, shouldbe tertiary alkyl groups. Tertiarybutyl radicals are especiallypreferred. The alkylphenyl groups may contain up to three alkylsubstituents. The alkyl groups preferably are bulky, i.e., tertiary orsecondary alkyl groups. Such alkyl groups include isopropyl, sec-butyl,tertiarybutyl, a-amyl, tertiaryamyl, n-hexyl, 2,2-dimethylbutyl,2-methyl-2-ethylpropyl, phenyl ethyl and tertiaryoctyl. The two alkylgroups are preferably in the 2, 4, or 6 positions or combinationsthereof in the 2,4-positions or 2,6-positions. A particularly preferredspecies is bis-(2,4-ditertiarybutylphenyl) pentaerythritol diphosphite.Another preferred species is bis-(2,6-ditertiarybutyl-4-methylphenyl)pentaerythritol diphosphite. Another especially desirably phosphite isbis(2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite.

The phosphite esters may be made by a variety of methods. Bis alkyl orAlkylAryl Pentaerythritol diphosphites may be prepared via the teachingsof U.S. Pat Nos. 4,305,866, 5,137,950, 4,064,100 or other meansdescribed in the literature.

Other phosphite antioxidants which can be employed includetrioctylphosphite, trilaurylphosphite, tridecylphosphite, octyldiphenylphosphite, tris(2,4-di-t-butylphenyl) phosphite,tris(nonylphenyl) phosphite, hexa(tridecyl)1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl) butane triphosphite,tetra(C₁₂₋₁₅ alkyl) 4,4'-isopropylidenediphenol diphosphite,tetra(tridecyl)4,4'-butylidenebis(3-methyl-6-t-butylphenol) diphosphite,hydrogenated-4,4'-isopropylidenediphenol polyphosphite, distearylpentaerythritol diphosphite, phenyl 4,4'-isopropylidenediphenolpentaerythritol diphosphite, bis(2,4-di-t-butylphenyl) pentaerythritoldiphosphite, bis(2,6-di-t-butyl-4-methylphenyl pentaerythritoldiphosphite, di(nonylphenyl) pentaerythritol diphosphite and4,4'-isopropylidenebis (2-t-butylphenol) di(nonylphenyl) phosphite.

Phenolic antioxidants which can be employed in the invention include,but are not limited to, 2,6-di-t-butyl-p-cresol,2,6-di-phenyl-4-octadecyloxyphenol,stearyl(3,5-di-t-butyl-4-hydroxyphenyl)-propionate,distearyl-3,5-di-t-butyl-4-hydroxybenzylphosphonate,thio-diethylenebis(3,5-di-t-butyl-4-hydroxyphenylpropionate,hexamethylenebis(3,5-di-t-butyl-4-hydroxyphenylpropionate,4,4'-thiobis(6-t-butyl-m-cresol),2-octylthio-4,6-bis(3,5-di-t-butyl-4-hydroxyphenoxy)-s-triazine,2,2'-methylenebis(4-methyl-6-t-butylphenol),2,2'-methylene-bis(4-ethyl-6-t-butylphenol),bis(3,3-bis(4-hydroxy-3-t-butylphenyl) butylic acid) glycol ester,4,4'-butylidenebis (6-t-butyl-m-cresol),2,2'-ethylidenebis(4,6-di-t-butylphenol),2,2'-ethylidenebis(4-sec-butyl-6-t-butylphenol),3,6-dioxaoctylenebis(3-methyl-5-t-butyl-4-hydroxyphenylpropionate),1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,bis(2-t-butyl-4-methyl-6-(2-hydroxy-3-t-butyl-5-methyl benzyl)phenyl)terephthalate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-t-butylbenzyl)isocyanurate, 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate,1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene,1,3,5-tris((3,5-di-tbutyl-4-hydroxyphenyl)propionyloxyethyl)isocyanurate,tetrakis(methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate)methane.

Metal salts of lactic acids, include metal salts of blends of lacticacid with other organic acids such as fatty acids. Preferred metal saltsof lactic acid include calcium lactate and calcium stearoyl lactate.Suitable metals include alkaline earth metals such as calcium, barium,stronium and radium. Suitable metals also include alkali metals such aslithium, sodium, potassium, rubidium, cesium, francium, cadium, lead,zinc, tin, magnesium and antimony. Preferably the metal is bivalent sothat the salt may be a co-salt of lactic acid and a fatty acid. Organicacids suitable for use in combination with the lactic acid includestearic acid, lauric acid, palmitic acid, butyric acid and other C₄ toC₂₂ fatty acids, including both saturated and unsaturated fatty acids,including linoleic and linolenic acids. Suitable fatty acids may berepresented by the formula: ##STR2## wherein W is a C₂ -C₂₀ saturated orunsaturated group. Saturated fatty acids may be represented by theformula: ##STR3## wherein x is selected from 2 to 27.

The preferred salts are bivalent metal co-salts of lactic acid and afatty acid, preferably a saturated fatty acid. The preferred bivalentmetal is calcium. The preferred saturated fatty acid is stearic acid.The preferred metal co-salt is calcium stearoyl lactate. Suitable otherco-salts of lactic acid include calcium lauroyl lactate, calciumpalmitoyl lactate and calcium butroyl lactate.

The polyolefin resin compositions preferably comprise from 50 to 99.9weight percent polyolefin resin, more preferably from 90 to 99.5 weightpercent thereof, and most preferably from 95 to 99 weight percentthereof based on the total weight of the composition; from 0.001 to 5weight percent phosphite, more preferably from 0.005 to 3 weight percentthereof, and most preferably from 0.025 to 1 weight percent thereofbased on the total weight of the composition; and preferably comprisesfrom 0.01 to 5 weight percent metal salt of lactic acid, more preferablyfrom 0.05 to 3 weight percent thereof, and most preferably from 0.05 to1 weight percent thereof based on the total weight of the composition.

The composition may also contain or be free of other additives such aswaxes, antistatic agents, flame retardants, nucleating agents,plasticizers, hindered amine light stabilizers, and hindered phenolicantioxidants. Optionally the composition contains a hindered phenolicantioxidant at a level of from 0.001 to 5 weight percent, morepreferably at a level of from 0.001 to 3 weight percent thereof, andmost preferably at a level of from 0.025 to 0.3 weight percent thereofbased on the total weight of the composition.

Polyolefin fibers are typically made by melt spinning processes. Meltspinning requires that the polyolefin polymers be stable at temperaturessufficiently above the melting point or softening point of thepolyolefin to be extruded in the molten state without substantialdegradation. The melt spinning process employs a spinneret, which is aplate containing orifices through which molten polymer is extruded underpressure. Typically the spinneret is made of stainless steel or a nickelalloy. The spinneret is a flat plat, flush with or recessed in itsmounting. Spinnerets for molten polymers are usually from 3 mm to 10 mmthick, for melt process pressures of up to 3000 psi. Fibers formingspinneret holes may have exit diameters of from 175 to 750 microns. Thenumber of holes in the spinneret may range from a few to severalthousand. A typical process is shown schematically in FIG. 1, whereinthe polyolefin composition in particulate form is fed via a hopper 10 toa screw type extruder 12 wherein the composition is melted at elevatedtemperatures to form a melt stream which is forced at elevated pressuresto a metering pump 14 which controls the flow. Optionally there may be afiltration unit (not shown) located at the exit of the extruder 12. Themelt stream is then forced through a filter 16, preferably a screen packfilter of filters in series (16^(i), 16^(ii), 16^(iii), 16^(iv), 16^(v))with the upstream filters being of a mesh for collecting only largeparticles and subsequent downstream filters being increasingly fine forcollecting smaller particles that pass through the upstream filters,which removes unmelted solids prior to the melt stream reaching thespinneret 18. The filtered use of stream is then forced to the spinneret18 wherein fibers are formed by passing the melt stream through the dieholes of the spinneret. The fibers are then air cooled and convergedinto the convergence guide 20, then directed to the finish application22, reels 24, 26, and finally to the spin bobbin 28 wherein the fiber iswound for storage.

As shown in FIG. 2, a film making process may involve feeding polyolefinparticulates (pellets or powder) to a hopper 24 of a screw type extruder26 wherein the particulates are melted and forced to a metering pump 28(optional) and then forced through a filtering system (preferably ascreen pack) 30 which preferably has a series of filters (30^(i),30^(ii), 30^(iii), 30^(iv) and 30^(v)) which have increasingly finesmesh as the polyolefic melt flows downstream. The filter screens out theunmelted solid by-products before the polyolefin melt stream reaches thedie 32 so that the dies orifice 33 will not become clogged by the solidby-products. The melt stream flows from the filter system 30 to the die32, through the elongated die orifice 33, forming a polyolefin filmwhich then passed partially around and between calendar rolls 34, 36 tostorage roll 38 whereupon the film is wound and stored.

As shown in the drawings, before reaching the spinneret, the moltenpolymer is filtered through a series of sintered or fibrous metal gauzesor a bed of graded fine refractory material, such as sand or alumina,held in place by metal screens. Filtration removes large solid or gelparticles that might otherwise block spinneret holes or, if passedthrough, occupy sufficient cross-sectional area in the filament toaffect its processing or tensile properties. Smaller particles, such asdelusterants, are not retained by the filter. Filtration also providesshearing, and thus can influence rheological behavior.

EXAMPLES

Example 1 was a polypropylene composition containing 500 parts permillion by weight (ppm) bis(2,4-di-tert-butylphenyl) pentaerythritoldiphosphite (sold under the trademark Ultranox 626 by GE SpecialtyChemicals Inc.), 250 ppm of a hindered phenolic compound (sold under thetrademark Irganox 3114 by B. F. Goodrich), 500 ppm Calcium StearoylLactate (sold under the trademark Pationic 930 by Patco PolymerAdditives Div., American Ingredient, Co.). The polyproylene base resinused in the compositions of the examples was Himont Profax 6301 resin.

Comparative Example 2 has a polypropylene composition as an Example 1,but 500 ppm of Calcium Stearate was used in place of the CalciumStearoyl Lactate.

Test method--Polypropylene is compounded with additives. Our laboratorycompound method 450° F. stock temperature using a 24:1 L/D 1" 2 stagescrew with a Maddox mixer between stages. A screen pack composed of20/100/500/100/20 mesh screens is utilized. Polypropylene is re-extrudedusing a 3/4" Brabender extruder with a single stage screw 2:1compression having a Maddox mixer 6" from the screw tip. The output ofthe extruder is throttled to a 1/4" diameter focus on a screen pack. Thescreen pack is composed of 20/100/1400×125/100/20 screens. Back pressureis set to 200-300 psi.

The extrusion is performed at 600° F. stock temperature operating theextruder at 10 rpm for 50 min and 50 rpm for 10 minutes out of everyhour. Back pressure is set at 200-300 psi. The extrusion is performeduntil significant pressure rise occurs or if none is observed for 13-16hrs.

                  TABLE 1                                                         ______________________________________                                        Examples         Ex 1         CE × 2                                    ______________________________________                                        Back Pressure Increase,                                                                        0(13 hrs)                                                                              +       500(8 hrs)                                  psi                                                                           ______________________________________                                    

Note that the examples of the present invention exhibited no backpressure increase after 13 hours of operation whereas the comparativeexample exhibited a +500 psi increase in back pressure after only 8hours.

Typical filter mesh sizes are from 20 mesh to 1000 mesh, for example, 20mesh, 100 mesh and 500 mesh. The higher the mesh number the finer thefiltration. The process of the present invention preferably employs afilter fineness of at least 20 mesh, more preferably at least 100 mesh,more preferably at least 500 mesh and most preferably in series of 20mesh, 100 mesh and 500 mesh so that the upstream filters filter out thelargest particles and the downstream filters filter out the fineparticles.

Example 3 was a polypropylene composition containing 800 ppmbis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, 500 ppmcalcium stearoyl lactate, and 250 ppm tris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate, and exhibited no screen pack plugging pressurerise during extended use.

Comparative Example 4 was a composition having the formation of Example3 except the calcium stearoyl lactate was replaced with calciumstearate, and exhibited a substantial rise in pressure during meltextrusion due to screen packplugging.

Example 5 was a polypropylene composition containing 600 ppmbis(2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite, 500ppm calcium stearoyl lactate. The composition exhibited no screen packplugging pressure rise during 13 hours of operation.

Comparative Example 6 is a polypropylene composition as in example 5that the calcium stearoyl lactate was replaced with calcium stearate,and the composition exhibited a 200 psi pressure rise in 7 hours ofoperation.

What is claimed is:
 1. A composition for melt extrusion which resistsscreen pack plugging, said composition comprisinga) a polyolefin presentat a level of from 50 to 99.9 weight percent based on the total weightof the composition, b) bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite present at a level of from 0.001 to 5 weightpercent based on the total weight of the composition, and c) a metalsalt of a lactic acid present at a level of from 0.01 to 5 weightpercent based on the total weight of the composition.
 2. The compositionas claimed in claim 1 wherein said metal salt of a lactic acid iscalcium stearoyl lactate.
 3. The composition as claimed in claim 1wherein said polyolefin is polypropylene.